bpf: reject program if a __user tagged memory accessed in kernel way

BPF verifier supports direct memory access for BPF_PROG_TYPE_TRACING type
of bpf programs, e.g., a->b. If "a" is a pointer
pointing to kernel memory, bpf verifier will allow user to write
code in C like a->b and the verifier will translate it to a kernel
load properly. If "a" is a pointer to user memory, it is expected
that bpf developer should be bpf_probe_read_user() helper to
get the value a->b. Without utilizing BTF __user tagging information,
current verifier will assume that a->b is a kernel memory access
and this may generate incorrect result.

Now BTF contains __user information, it can check whether the
pointer points to a user memory or not. If it is, the verifier
can reject the program and force users to use bpf_probe_read_user()
helper explicitly.

In the future, we can easily extend btf_add_space for other
address space tagging, for example, rcu/percpu etc.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154606.654961-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/bpf/bpf_dummy_struct_ops.c b/net/bpf/bpf_dummy_struct_ops.c
index fbc896323bec9..d0e54e30658ac 100644
--- a/net/bpf/bpf_dummy_struct_ops.c
+++ b/net/bpf/bpf_dummy_struct_ops.c
@@ -145,7 +145,8 @@ static int bpf_dummy_ops_btf_struct_access(struct bpf_verifier_log *log,
 					   const struct btf *btf,
 					   const struct btf_type *t, int off,
 					   int size, enum bpf_access_type atype,
-					   u32 *next_btf_id)
+					   u32 *next_btf_id,
+					   enum bpf_type_flag *flag)
 {
 	const struct btf_type *state;
 	s32 type_id;
@@ -162,7 +163,8 @@ static int bpf_dummy_ops_btf_struct_access(struct bpf_verifier_log *log,
 		return -EACCES;
 	}
 
-	err = btf_struct_access(log, btf, t, off, size, atype, next_btf_id);
+	err = btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+				flag);
 	if (err < 0)
 		return err;